Close persistence.xml after reading it

Using `hibernate-jpamodelgen-1.3.0.Final.jar` together with Eclipse Annotation Processing causes a problem. After the generation the project can't be cleaned any more. The error is: 

```
The project was not built due to "Could not delete '/Project1/bin/META-INF'.". Fix the problem, then try refreshing this project and building it since it may be inconsistent
```

It turns out, that the problem is unclosed persistence.xml file read by the generator. Closing it fixes the problem.